Transport of mannitol in Streptococcus mutans has been shown to be mediated by a phosphoenolpyruvate phosphotransferase system in which at least one component is inducible. Several classes of mutants of S. mutans have been isolated which fail to grow on mannitol but grow on glucose and other carbohydrates. One class is inhibited by the addition of mannitol to cultures growing in the presence of glucose. The physiological basis of this sensitivity to mannitol is under investigation. A second class of mannitol-negative mutants of S. mutans agglutinates rapidly in the presence of sucrose. Mutants of this type adhere poorly to glass surfaces and possess alterations in glucosyltransferase activity. The biochemical mechanism of this change in glucosyltransferase activity is being studied to determine its significance to this organism's ability to colonize and cause caries in animals.